1. Technical Field
The present invention relates to a semiconductor device and a booster circuit, and is particularly preferable to apply to a switched-capacitor DC-DC converter.
2. Related Art
In recent years, accompanying the developments of semiconductor manufacturing processes, semiconductor integrated devices have been integrated in highly density and become multifunctional. As a result, a power supply voltage inside a semiconductor integrated device circuit tends to be lowered. In addition, various power supply sources including high voltage power supply sources have been incorporated as the semiconductor integrated circuits become multifunctional. For example, a high voltage of 10 V or more is required in nonvolatile memories such as flash memories, and EEPROMs, and driver ICs for display elements such as liquid crystal displays. Thus, a charge pump method, which is easily incorporated into the semiconductor integrated devices, is employed as a booster circuit to generate such high voltage instead of a switching regulator method using coils and so on. As the charge pump method, Dickson charge pump circuit is typically used.
In order to achieve higher conversion efficiency than the charge pump circuit, a method using a DC-DC converter that employs a switched-capacitor method, i.e. switched-capacitor DC-DC converter, as a booster circuit is disclosed in JP-A-2004-172631. In the switched-capacitor method, a plurality of kick capacitors to which power supply voltage is applied parallel, is switched to be connected in series by a switching element, thereby a boosted output voltage can be output.
However, in a case where a kick capacitor used for the switched-capacitor method is structured by a MOS capacitor, a problem arises in that boosted voltage is lowered since a depletion layer capacitance formed along a bonded surface of a well and a substrate acts as a parasitic capacitance.
In contrast, in a case where the kick capacitor is structured by a SiO2/Si3N/SiO2 (ONO) capacitor having a polysilicon layer as its upper and lower electrodes, a problem arises in that an element area increases.